Scientists pinpoint origin of eerily familiar sensation

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The brain cranks out memories near its center, in a looped wishbone of tissue called the hippocampus. But a new study suggests only a small chunk of it, called the dentate gyrus, is responsible for “episodic” memories — information that allows us to tell similar places and situations apart.

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The finding helps explain where déjà vu originates in the brain, and why it happens more frequently with increasing age and with brain-disease patients, said MIT neuroscientist Susumu Tonegawa. The study is detailed today in the online version of the journal Science.

Like a computer logging its programs’ activities, the dentate gyrus notes a situation’s pattern—it’s visual, audio, smell, time and other cues for the body’s future reference. So what happens when its abilities are jammed?

When Tonegawa and his team bred mice without a fully-functional dentate gyrus, the rodents struggled to tell the difference between two similar but different situations.

“These animals normally have a distinct ability to distinguish between situations,” Tonegawa said, like humans. “But without the dentate gyrus they were very mixed up.”

Déjà vu is a memory problem, Tonegawa explained, occurring when our brains struggle to tell the difference between two extremely similar situations. As people age, Tonegawa said déjà-vu-like confusion happens more often—and it also happens in people suffering from brain diseases like Alzheimer’s. “It’s not surprising,” he said, “when you consider the fact that there’s a loss of or damage to cells in the dentate gyrus.”

As an aging neuroscientist, Tonegawa admitted it’s a typical phenomenon with him. “I do a lot of traveling so I show up in brand new airports, and my brain tells me it’s been here before,” he said. “But the rest of my brain knows better.”